DAI is a known hair dye. It is an expensive material because all prior art syntheses of same involve difficult multi-step procedures, usually involving from 5 to 8 steps. Moreover, extensive purification is required in order to isolate a good quality DAI. Because of the high expense the aforementioned prior art syntheses have not proven suitable from an industrial standpoint.
R. J. S. Beer, et al (J.Chem.Soc.223,1948) teach production of DAI by reductive cyclization of 4,5-diacetoxy-2,.beta.-dinitrostyrene with Fe in acetic acid. 10 g of Fe, 40 ml of acetic acid and 50 ml of absolute alcohol were required for reduction of 2 g of 4,5-diacetoxy-2,.beta.-dinitrostyrene. Moreover, in order to isolate DAI from the reaction mixture, several extractions with ether (at least 5 times), and recrystallization, were necessary.
B. P. Murphy (J.Org.Chem.,50,5873, 1983) teaches that DAI may also be obtained by reductive cyclization of 4,5-diacetoxy-2,.beta.-dinitrostyrene with 5% Pt/C, in acetic acid, followed by acetylation. The process required 5 operations. Moreover, it was necessary to use HPLC to purify DAI so produced.
In U.S. Pat. No. 3,732,245, issued May 8, 1973, Batcho et al disclose a process and intermediates for the preparation of indoles from ortho-nitrotoluenes. The process comprises condensing an ortho-nitrotoluene with formamide acetal then treating the reaction product with a reducing agent to yield the indole. At column 4 patentees indicate that their invention involves condensing the methyl function of an ortho-nitrotoluene with the formyl group of a formamide acetal to produce a nitrobenzene derivative bearing a N,N-disubstituted aminovinyl function ortho to the nitro group. The nitro group is reduced to an amino group which concurrently displaces the N,N-disubstituted amino function and effects cyclization to a compound having an indole nucleus.
The synthesis described by Batcho et al is a multi-step process. The final product, 5,6-dibenzyloxyindole, was purified by a silica gel column chromatography followed by recrystallization. Although the patent does not suggest or indicate the synthesis of DAI by this route, one can visualize that DAI can be obtained from 5,6-dibenzyloxyindole by a two-step procedure, debenzylation and subsequent acetylation.